In Lisbon we usually stay at Residencial Florescent in the Baxia area. It's located on a pedestrian street within walking distance to all forms of transportation, restaurants, the famous Rua Augusta, and many sights. We've been to Portugal many times and have used RS guide book as well as on line forums. Some of the many things to see in Lisbon are the Tower of Belem, the Discoveries Monument, the Jeronimos Monastery, the Coaches Museum, and the cafe Pasteis de Belem for the famous pastries, all of which are in close proximity. You can also see and ride up the Elevador Santa Justa, visit the Castelo de Sao Jorge, the old Cathedral known as Se, and the Gulbenkien Museum. Other Lisbon sights include the church and Monestary of Sao Vicente de Fora with it's beautiful blue tiles; the church and museum of Sao Roque with it's beautiful lapis lazuli altar; the Botanical Garden; the tile museum and Church Madre de Deus; coffee and pastry at the famous Cafe a Brasileira; One must day trip is to visit Sintra, especially the Palacio da Pena, but Sintra also has other palaces and museums to visit. Other day trips might be to Queluz, a palace modeled on Versailles, and to Casilhas via ferry across the river where you can visit the famous statue of Christ and have beautiful views back to Lisbon.
You can google these sights and others, and decide upon your itinerary. You didn't say when you plan to go or for how long so it's bit difficult to suggest what to see or do. As you can see, we love Lisbon and Portugal and hope you will, too!